The invention is concerned with a process for dehydration and/or desalination and simultaneous fractionation of a petroleum deposit effluent containing oil, associated gas and water which can be saline.
Treating a petroleum deposit effluent usually comprises an oil-gas fractionation step which is intended to adjust the droplet point of the oil produced in order to make it thermodynamically stable under storage conditions.
The treatment also generally comprises a step in which an aqueous liquid phase which is not miscible with the oil phase is separated and which may possibly both be saline. In fact, an aqueous phase is usually produced at the same time as the liquid and gaseous hydrocarbons, and must be removed in order to satisfy water content requirements and salt content requirements in the oil produced. Moreover, to reduce the content of salt in the oil produced, it is often necessary to remove salt from the oil by mixing the oil with fresh water or water whose salt content is less than that of the water produced at the same time as the oil, then separating the liquid phases of water and oil.
The treatment can also comprise a de-acidification step of the oil produced, this step consisting mainly of extracting the major part of the sulfurated hydrogen (H.sub.2 S) for toxicity and corrosion related reasons.
These steps are usually simultaneous, the production effluent undergoing a series of successive expansions, usually 3 or 4, from the pressure at which the effluent issues from the well to a pressure close to atmospheric pressure, with, on each expansion, removal or recompression of the gas produced and removal of the aqueous phase by decantation possibly with fresh water being injected between expansions to produce a desalination effect. The effluent is usually heated before the last expansion in order to satisfy vapor pressure requirements and H.sub.2 S requirements of the oil produced, and to facilitate separation of the aqueous phase.
For the oil produced, the necessary requirements are as follows:
For vapor pressure, the criterium usually held is the vapor pressure at 37.8.degree. C. expressed in Pascals, or Reid Vapor Pressure (RVP). This requirement is usually between 8 and 12 p.s.i. PA1 For the H.sub.2 S content, the usual requirement is 60 ppm by weight. PA1 For the water content, the requirement is usually between 0.5 and 1%. PA1 It has also been discovered that these treatment operations can be carried out in at least one expansion step unless it is normal practice to carry out the fractionation operations by way of successive expansions. PA1 (a) at least one step where the liquid and gaseous phases are separated at the pressure P1 for removal of the gas, producing, on the one hand, a gaseous fraction G1 which is removed, and, on the other hand, a liquid fraction L1 which is sent to step (b); PA1 (b) at least one step where the two liquid phases mixed in the liquid fraction L1 are at least partly separated, the aqueous phase being partly removed and the oil phase which contains a residual quantity of aqueous phase being sent to step (c), PA1 (c) at least one distillation step which is carried out at a pressure P2 which is less than or at the most equal to the pressure P1 in step (a), in a distillation zone C1, said distillation being carried out in the presence of the oil phase coming from step (b), said zone C1 comprising an internal heat exchange zone and a boiling zone, and enabling a gaseous fraction G2 to be collected, on the one hand, and a liquid fraction L2 to be collected, on the other hand, said fraction L2 being constituted of a liquid oil phase and of an aqueous liquid phase which is not miscible with the liquid oil phase, and PA1 (d) at least one step For separating the two liquid phases which are mixed in the liquid Fraction L2, the oil phase being sent to said internal exchange zone, and then removed.
The gas produced on each expansion contains heavy constituents (C.sub.4.sup.+) which it is not necessary to draw off from the oil to stabilize it. It is desirable to recover these heavy constituents to reinject them into the oil because this increases the amount of stabilized oil produced, whilst lowering its density which will increase its selling value. Moreover, the presence of these heavy constituents in the gas produced confers upon it a high hydrocarbon dew point which is unfavourable in terms of its marketing.
Recompression of the gas produced on each expansion and also partial condensation thereof by cooling allows a part of the heavy constituents to be recovered, but also excessively increases investment costs as the amount of compression needed increases.